1. Field of the Invention
The present invention relates to an image pickup lens for forming an image of an object on a solid imaging element such as a CCD sensor or a C-MOS sensor adopted in small-sized imaging devices. More specifically, the present invention relates to an image pickup lens composed of five lenses, which is built into an imaging device mounted on portable terminals such as cellular phones and smartphones, PDAs (Personal Digital Assistances), and game machines or information terminals such as personal computers and the like, where downsizing and thinning are pursued.
2. Description of the Related Art
Recently, the market of portable terminals having imaging devices is expanding more and more. Most portable terminals are equipped with a camera function, and currently, the majority of such camera functions has a large number of pixels comparable to that of digital cameras. Along with the increasing demands for thinning of portable terminals for reasons such as user-friendliness and design, demands for downsizing and thinning of the imaging devices built therein are also becoming severe. Further, for the image pickup lens mounted on imaging devices adopting such imaging elements having a large number of pixels, it is demanded to be even higher resolution, downsized, thinner and brighter (that is, with a small F-value). Also, it is strongly demanded for an imaging lens to have a wide angle of field suitable for taking an image in a wide area. In order to answer to the trend of downsizing, thinning and enhanced performance, the image pickup lens is usually composed of multiple lenses. Conventionally, image pickup lenses having a two or three lens configuration have been widely adopted for a VGA class to one-megapixel-class lens, because of an advantage in terms of its size and cost. Further, in order to adapt for increasing the number of pixels, many four-lens configuration image pickup lenses have been proposed. However, in order to cope with further downsizing and increase in the number of pixels, many image pickup lenses having a five-lens configuration, achievable a higher performance than the four-lens configuration, have been proposed. The present invention corresponds to such five-lens configuration.
For example, Japanese Patent Laid-Open No. 2009-294528 (Patent Document 1) discloses an image pickup lens having a five-lens configuration composed of, in order from an object side, a first lens having a positive power with an object side surface formed as a convex surface, a stop, a second lens having a meniscus shape near an optical axis, a third lens having an image side surface formed to have a convex shape near the optical axis, a fourth lens having both sides thereof formed as aspheric surfaces and where a circumference portion of an image-side surface is formed to have a convex shape, and a fifth lens having both sides thereof formed as aspheric surfaces and where a circumference portion of an image-side surface is formed to have a convex shape, wherein only one of the second to fifth lenses is a negative lens having an Abbe number of 30 or smaller.
Further, Japanese Patent Laid-Open No. 2010-026434 (Patent Document 2) discloses an image pickup lens having a five-lens configuration composed of, in order from an object side, a positive first lens, a positive second lens, a negative third lens, a positive fourth lens, and a negative fifth lens.
According to the image pickup lens disclosed in Patent Document 1, a ratio (TTL/2IH) of a total track length (TTL) to a maximum image height (IH) is approximately 1.0, so that relative downsizing of the lens is realized. However, the F-value of the lenses is approximately 3.0, which cannot be recognized as ensuring a sufficient brightness for imaging elements having increased number of pixels. As for the image pickup lens disclosed in Patent Document 2, it realizes a lens system having an F-value as bright as 2.05 to 2.5 and high aberration correction ability, but since the power of the first lens is weak, the configuration is disadvantageous for realizing a thinner lens.